Natural rubber from the guayule shrub, Parthenium argentatum, is similar in structure and physical properties to natural rubber from the Hevea brasiliensis tree. These natural rubbers have a linear cis-polyisoprene structure that cannot be readily manufactured by a polymerization process. Natural rubber is used in tires for its low rate of wear and heat hysteresis to avoid premature tire damage. For this and other applications there is no suitable substitute for natural rubber. Guayule can actually provide a purer grade of natural rubber than Hevea, depending on the technology used to recover the rubber from the shrub. Guayule rubber is soluble in hydrocarbon, aromatic, clorinated hydrocarbon, ether and other lipophilic solvents. Hevea rubber must be milled to increase its solubility, which also breaks down its molecular weight.
An attractive feature of guayule is that the shrub grows in semi-arid lands of the southwest United States, and is indigeneous to certain areas of Texas and Mexico. Large marginal land areas could be made productive with this shrub. At one time there were as many as twenty factories producing rubber in Mexico from wild stands growing in the northern part of that country. Up through 1946 the United States imported more than 150 million pounds of Mexican guayule rubber. During the 1920's and again in the 1940's guayule was cultivated in California and Texas, and processed into rubber. All of the processing facilities used a wet milling flotation process that was water intensive and inefficient. Resin content of the rubber was a problem and the wet process facilitated oxidation of the rubber to lower its quality. In addition to providing a less pure rubber, the wet process did not maximize the resin value of the shrub, and removed potentially nutritious components such as carbohydrates and protein from the bagasse. The wet process requires defoliation prior to rubber processing in order to minimize oxidative degradation of the rubber catalyzed by metal ions in the leaves. This traditional process does not optimize rubber, resin and bagasse values which is necessary for an economically viable guayule industry.
At present there are no operational guayule processing facilities in the United States or Mexico. The guayule plant is a tough woody shrub and the rubber polymer occurs inside the cells. New and improved technology is necessary to process this recalcitrant shrub. The processes described herein are designed to shorten processing time, maximize high purity rubber recovery, preserve physical properties and optimize by-product values.